The present invention relates to technology for time synchronization between devices in a communication system. Specifically, it relates to technology for a transmission device connected with a terminal to convey time information to the terminal.
Time distribution services having been conventionally offered to general users include time distribution by use of radio waves, notification services by use of time, and services by use of the Internet. A typical example of the application of time distribution technology by use of radio waves is a wave clock. As time distribution on the Internet, for example, a time synchronization method by use of NTP (network time protocol) is available.
Major applications of traditional standard time distribution have been synchronization between transmission devices, the management of fault occurrence time, and the use (keeping the consistency of time) of time stamp in the exchange of mail, data, and the like. Time accuracy in standard time distribution by use of NTP and existing phone lines is no more than several milliseconds. This is a sufficient accuracy in time adjustment of individual PCs and servers. This is because, in the range of traditional applications, an object has been to independently confirm each of the times of devices existing on a network. For example, although mail arrival time is highly different from transmission time due to the influence of transmission delay and the like on the network, the difference is no problem in terms of use. In downloading of Web data, inherently, it has not been necessary at all to take PC setting time of individual users into account.
The construction of an optical access network has been promoted for general users. One of its representative system is a G-PON (Gigabit Capable Passive Optical Network)(ITU-T Recommendation G.984.1 (2003), Gigabit-capable Passive Optical Networks (G-PON): General Characteristics, ITU-T Recommendation G.984.3 (2004), Gigabit-capable Passive Optical Networks (G-PON): Transmission convergence layer specification, ITU-T Recommendation G.984.3 Amendment 1 (2005), Gigabit-capable Passive Optical Networks (G-PON): Transmission convergence layer specification) standardized in ITU-T. Since G-PON uses basic cycle frames of 125 microseconds for data transmission control, it can accommodate information distribution at a fixed rate and at a fixed cycle such as E1 and T1 lines (specifications of high speed digital line) having been conventionally used in private line services, and best-effort type communications of variable bands as typified by Ethernet (registered trademark) having been used in data communications. A PON is a system that is being promoted for its introduction as users of the Internet increase. The PON is in the form of PDS (Passive Double Star) using a passive optical element, and accommodates plural user terminals at the same time for one base station device (transmission device). Multiplexing communications of each user on an identical optical fiber has the advantages of facilitating user management (increase or decrease in facilities, fault monitoring, etc.) and reducing optical fiber installation costs.